Studies on prolactin 48: isolation and properties of the hormone from horse pituitary glands

A xenotransplantation mouse model to study physiology of the mammary gland from large mammals

Although highly conserved in structure and function, many (patho)physiological processes of the mammary gland vary drastically between mammals, with mechanisms regulating these differences not well understood. Large mammals display variable lactation strategies and mammary cancer incidence, however, research into these variations is often limited to in vitro analysis due to logistical limitations. Validating a model with functional mammary xenografts from cryopreserved tissue fragments would allow for in vivo comparative analysis of mammary glands from large and/or rare mammals and would improve our understanding of postnatal development, lactation, and premalignancy across mammals. To this end, we generated functional mammary xenografts using mammary tissue fragments containing mammary stroma and parenchyma isolated via an antibody-independent approach from healthy, nulliparous equine and canine donor tissues to study these species in vivo. Cryopreserved mammary tissue fragments were xenotransplanted into de-epithelialized fat pads of immunodeficient mice and resulting xenografts were structurally and functionally assessed. Preimplantation of mammary stromal fibroblasts was performed to promote ductal morphogenesis. Xenografts recapitulated mammary lobule architecture and contained donor-derived stromal components. Mammatropic hormone stimulation resulted in (i) upregulation of lactation-associated genes, (ii) altered proliferation index, and (iii) morphological changes, indicating functionality. Preimplantation of mammary stromal fibroblasts did not promote ductal morphogenesis. This model presents the opportunity to study novel mechanisms regulating unique lactation strategies and mammary cancer induction in vivo. Due to the universal applicability of this approach, this model serves as proof-of-concept for developing mammary xenografts for in vivo analysis of virtually any mammals, including large and rare mammals.

Glycosylated equine prolactin and its carbohydrate moiety

Glycosylated equine prolactin (G-ePRL) and nonglycosylated ePRL were purified to homogeneity from side fractions obtained during isolation of LH/FSH from horse pituitaries. Both PRL forms were isolated together in high yield by the isolation procedure used for glycosylated porcine PRL/(G-pPRL) and pPRL, involving acetone extraction/precipitation, NaCl and isoelectric precipitation, and gel filtration. Purification of G-ePRL required additional Con A chromatography. The N-terminal amino acid sequencing for 32 cycles of G-ePRL and ePRL resulted in sequences identical to the known primary structure of ePRL. Based on MALDI mass spectrometry analysis and SDS-PAGE mobilities, G-ePRL and ePRL had estimated molecular weights of 25,000 and 23,000 Da, respectively. G-ePRL displayed only 60% of the immunoreactivity of ePRL in homologous radioimmunoassay. Using the Nb2 lymphoma cell bioassay, ePRL was found to have about 1/30th the mitogenic activity of bovine PRL; G-ePRL was approximately 1/10th as active as ePRL. Glycosylation of G-ePRL at Asn31 was confirmed by isolation and sequence analysis of an enzymatically derived G-ePRL glycopeptide spanning residues 29-37. Monosaccharide compositions of intact G-ePRL and this glycopeptide were very similar (Man3, GlcNAc2, GalNAc1, Fuc0.6, Gal0.2, NeuAc0.15) and resembled that of G-pPRL. The glycopeptide contained one sulfate residue as determined by ion chromatography after acid hydrolysis, indicating the presence of a sulfated monosaccharide. Comparative carbohydrate analysis of G-ePRL and other G-PRL preparations suggests that the functionally significant Asn31 carbohydrate unit is a fucosylated complex mono- and /or biantennary oligosaccharide terminating with a sulfated GalNAc residue and two or three Man residues.

Molecular cloning of silver carp and bighead carp prolactin

The cDNAs encoding the prolactin of silver carp (scPRL) and bighead carp (bcPRL) have been cloned. Deduced from the nucleotide sequences, both scPRL and bcPRL are composed of 187 amino acid residues. Only one residue is different between scPRL and bcPRL. Homology analysis indicates that scPRL and bcPRL are highly homologous to carp PRL (97%), relatively conserved in relation to PRLs of salmon, trout, and tilapia (64-69%), and diversified from avian and mammalian PRL (30-35%). Similar to PRLs of other species of fish, scPRL and bcPRL lack the first 12 N-terminal residues of avian and mammalian PRLs.

Determination of the primary and secondary structures of the dromedary (Camelus dromedarius) prolactin and comparison with prolactins from other species

A non-glycosylated form of camel prolactin (camPRL), isolated from one-humped camel (Camelus dromedarius) pituitaries, was totally sequenced. A glycosylated form, separated by affinity chromatography on ConA-Sepharose, was partially sequenced. The comparison of the N-terminal amino acid sequences of the glycosylated and non-glycosylated forms showed that the only putative site of N-glycosylation (Asn-31) was indeed glycosylated. The far ultraviolet (UV) circular dichroism (CD) spectra of the two isohormones were identical, suggesting that the carbohydrate moiety had no effect on the global camPRL secondary structure. The far UV circular dichroism spectra of the two isohormones were analyzed in order to determine their relative proportions of periodic secondary structure, 60% of which was found to be in alpha-helix, as in prolactins of other species. The dromedary sequence was compared to those of other species and interpreted in term of evolutionary process. As already found for gonadotropins, the closest species to the dromedary was found to be the pig.

Primary structure of equine pituitary prolactin

Equine prolactin was determined to be a single chain protein of 199 amino acid containing two tryptophan and six cysteine residues, as found in other mammalian prolactins. The primary sequence of equine prolactin was obtained by automated Edman analyses of S-carboxymethylated protein and proteolytic fragments of modified protein. Of the known prolactin sequences, equine prolactin shows closest homology with porcine (93%) and fin whale (87-91%) prolactins. Genetic mutations have produced changes in 17 of 199 residues of equine prolactin relative to its putative ancestral precursor. Since equine growth hormone has undergone alterations in 4 of 191 residues relative to this putative precursor protein, these results support the theory that prolactins are evolving at a faster rate than growth hormones. Consistent with the previously determined circular dichroic spectrum of equine prolactin, 60% of the protein is predicted to form alpha helices.

Conformation of equine pituitary somatotropin

Equine pituitary somatotropin (growth hormone) has been studied by zero-order and second-order absorption spectroscopy, and by circular dichroism. Difference absorption spectra have also been generated during proteolytic digestion of the hormone. The molar extinction coefficient of the native protein was found to be 16,050 +/- 330 M-1 cm-1 at 278.1 nm. Comparison of the conformations of equine somatotropin and somatotropins isolated from several other mammalian species indicates a close structural relationship between these molecules. With the increasing number of species which have been studied, it is becoming evident that with regard to conformation, the somatotropins can be subdivided into at least three major groups.

Elephant prolactin: isolation and characterization

Prolactin was isolated from anterior lobes of elephant pituitary glands. It consisted of 199 amino acids with three disulfide bridges and two tryptophan residues as found in prolactin from other species. The sequence of the NH2-terminal 28 amino acids was determined and shown homologous with the ovine hormone. In comparison with ovine prolactin, a marked difference was seen in the methionine content; the elephant hormone possessed only 18-34% lactogenic potency. The conformation of elephant prolactin was examined by zero order, second order and circular dichroism spectroscopy. The alpha helical content was estimated to be about 60%. In comparison with prolactins from other species, the second order spectra of elephant prolactin suggest that the local microenvironment for one or both tryptophan residues is somewhat different.

Purification and characterization of toad prolactin

Prolactin (PRL) was purified from anterior pituitary glands of adult toad (Bufo japonicus formosus) by extraction of acetone-dried pituitary powder with acid acetone and chromatography on DEAE-cellulose and Sephadex G-100. Purification was monitored by radioimmunoassay developed for bullfrog PRL. Toad PRL thus obtained is as potent as bovine PRL in promoting collagen synthesis in the bullfrog tadpole tail fin. HPLC of the toad PRL on a reverse-phase column shows that the preparation behaves as a homogeneous protein. The hormone has a molecular weight of 23,000 as determined by SDS gel electrophoresis. The isoelectric point is 5.5 as determined by analytical electrofocusing. The amino acid composition closely resembles that of bullfrog PRL.

Primary structure of chum salmon prolactins: occurrence of highly conserved regions

The complete amino acid sequence of prolactin from the pituitaries of salmon (Oncorhynchus keta) has been determined. Salmon prolactin comprised two variants, I and II, which were separated by reverse-phase high-performance liquid chromatography. Each variant was reduced, S-carboxymethylated, and then cleaved with cyanogen bromide and enzymes. The resulting fragments were separated by reverse-phase high-performance liquid chromatography, as well as gel filtration, and subjected to sequence analysis by the dansyl-Edman method. Both variants contain 187 amino acid residues with two disulfide linkages at residues 46-160 and 177-187, lack a linkage in the N-terminal portion of mammalian prolactins, and differ from each other by the replacement of only four amino acid residues. Salmon prolactin (sPRL) shows 31% sequence identify with ovine prolactin. Moreover, four restricted regions, i.e., sPRL (3-21), (46-60), (68-83), and (160-178), encompass this significant conservatism between the teleost and the mammalian hormone, with identities of 47, 87, 62, and 68%, respectively. Such considerable identity between these distant phylogenic species strongly suggests that these regions may be responsible for the biological activity of prolactin.

Isolation and properties of sea turtle (Chelonia mydas) pituitary prolactin

Sea turtle prolactin (PRL) was isolated in a highly purified state from sea turtle pituitary side fractions obtained from other studies and some of its biological, chemical, and immunological properties were determined. Sea turtle PRL is a protein of 22-24 kDa [sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis]. Its sole amino terminal amino acid residue is leucine. The amino acid composition of sea turtle PRL is similar to ovine PRL and is characterized by a high content of aspartic acid (20 residues), glutamic acid (34 residues), serine (19 residues), and leucine (24 residues). It possess three disulfide bonds and 2 tryptophan residues which is also characteristic of many species of PRL. As with PRLs of other species, it displayed multiple stained bands in disc gel electrophoresis, at pH 8.3. Biologically, sea turtle PRL was active in pigeon crop-sac assay but the dose-response characteristics were nonparallel when compared to ovine PRL. An antiserum against sea turtle PRL was raised in rabbit and a homologous radioimmunoassay was developed with a sensitivity of 2.8 ng for sea turtle PRL. Sea turtle gonadotropins did not cross-react, but sea turtle growth hormone showed a 5% cross-reactivity. Pituitary extracts from other species of turtles displayed parallel inhibition curves to the sea turtle PRL. Extracts and prolactin preparations from several birds, snakes, alligator, and marsupials cross-reacted, but in a nonparallel fashion. Bullfrog pituitary extract and Tilapia PRL showed no cross-reaction at high doses. Several purified mammalian PRLs (pig, sheep, human, horse, dog) showed minimal or no ability to cross-react in the RIA.

Studies on prolactin: conformational comparison of human, equine, and porcine pituitary prolactins

The conformations of human, equine, and porcine pituitary prolactins, as evidenced by various optical properties, have been compared. The alpha-helix contents of all three proteins are essentially identical to each other (60 +/- 5%), as well as to prolactins isolated from other mammalian species. Direct absorption (zero and second-order), difference absorption, fluorescence emission, and circular dichroism spectra suggest that the majority of tyrosine and tryptophan side chains in these three proteins exist in very similar microenvironments within the folded forms of the hormones. Thus, the general conformational properties of these molecules are closely related to each other, and to other mammalian prolactins. Molar extinction and absorptivity values have been obtained at the absorption maximum of each species. In addition, a second molar extinction value has been determined at a particular wavelength found to be different for each, and which appears to be independent of the conformational state of the molecule. These absorptivities are useful in providing accurate prolactin concentrations in the 10(0) to 10(-2) mg/ml range. On incubation with the proteolytic enzyme thermolysin, all three hormones display an initial, short lag period during which little conformational change can be detected by difference absorption spectroscopy. For human and porcine prolactins, subsequent rates of proteolytically induced conformational collapse were found to be essentially identical. However, under similar conditions, equine prolactin loses its conformation significantly more slowly.